The invention relates to a dielectric composition comprising a mixture of
a ceramic composition containing BaaREbTicO3, wherein RE represents a rare earth element, with 0.05xe2x89xa6axe2x89xa60.25, 0.525xe2x89xa6bxe2x89xa60.70, 0.85xe2x89xa6cxe2x89xa61.0, and 2a+3b+4c=6, and free from lead and bismuth, and
a glass composition comprising SiO2.
The invention also relates to a method of manufacturing a ceramic multilayer element comprising the steps of
manufacturing a multilayer stack comprising a first ceramic foil, a first electrode comprising Cu, a second ceramic foil, and a second electrode comprising Cu, which ceramic foils are manufactured from a dielectric composition comprising a ceramic composition and a glass composition comprising SiO2, which ceramic composition contains BaaREbTicO3, wherein RE represents a rare earth element, with 0.05xe2x89xa6axe2x89xa60.25, 0.525xe2x89xa6bxe2x89xa60.70, 0.85xe2x89xa6cxe2x89xa61.0, and 2a+3b+4c=6, the ceramic composition being free from lead and bismuth; and
sintering the multilayer stack.
The invention further relates to an electronic device comprising a first dielectric ceramic layer, a first electrode comprising Cu, and a second electrode.
Such a dielectric composition is known from EP-A-0926107. The ceramic composition BaaREbTicO3, wherein RE represents a rare earth element, with 0.05xe2x89xa6axe2x89xa60.25, 0.525xe2x89xa6bxe2x89xa60.70, 0.85xe2x89xa6cxe2x89xa61.0, and 2a+3b+4c=6, is generally known as a group of materials with a high dielectric constant. If no glass composition is added, however, the ceramic composition has a high sintering temperature, generally of more than 1300xc2x0 C. It is known that the addition of a glass composition leads to a decrease in the sintering temperature. The known dielectric composition has a sintering temperature of about 1000xc2x0 C., which is below 1084xc2x0 C., the melting temperature of copper. Since glasses themselves have a low dielectric constant, the composite dielectric composition comprising the ceramic composition and the glass composition has a relatively low dielectric constant as well; the relative dielectric constant is less than 50. In order to get a higher dielectric constant, lead or bismuth are generally present in the ceramic composition. The dielectric composition can be sintered at a low temperature. The presence of lead or bismuth, however, makes the dielectric composition toxic and harmful to the environment when incinerated or otherwise disposed of as waste.
The known dielectric composition further contains CuO. The presence of CuO gives the known dielectric composition a higher dielectric constant. The presence of CuO also results in a lower sintering temperature. This CuO is added as a powder or as an ingredient of the glass composition. If a Pb-containing ceramic composition is used, the relative dielectric constant is 62 without the presence of CuO and 77 with the presence of CuO, cf. the compositions nos. 2, 3, and 6 listed in the cited application.
Compositions, with nos. 21, 22, and 34 are known from the cited application which are free of lead and bismuth and which can be sintered at a temperature lower than 1100xc2x0 C. The glass composition in such a dielectric composition comprises 23% by weight of SiO2, 14% by weight of B2O3, 61% by weight of alkaline earth metal oxides, and 2% by weight of Li2O. The alkaline earth metal oxide comprises 82% by weight of BaO and at least one compound selected from the group consisting of SrO, CaO and MgO. The dielectric compositions referred to further contain CuO. The dielectric compositions have a firing temperature of 1000xc2x0 C. and relative dielectric constants of between 27 and 60.
Next to a high dielectric constant, it is necessary for electronic applications in the high frequency domain that the temperature coefficient of the dielectric constant should be small. A criterion for this is the NP0-standard, which states that the temperature coefficient of the dielectric constantxe2x80x94also known as the temperature coefficient of capacitance or TCCxe2x80x94should be in the range of xe2x88x9230 to +30 ppm/xc2x0 C. The cited dielectric compositions fulfilling this criterion and free of lead and bismuth have relative dielectric constants between 48 and 53.
According to this NP0-standard, furthermore, the dielectric composition should have an RC-time of at least 1000 seconds after sintering. This RC-time has been defined as the product of the resistance against insulation and the capacitance, and is a measure for the stability of the dielectric composition.
A disadvantage of said known dielectric compositions, which are free of lead and bismuth and of which the TCC fulfills the NP0-standard, is that the relative dielectric constant is relatively low. Dielectric compositions with high dielectric constants are required for the miniaturization of devices for high-frequency applications.
It is therefore a first object of the invention to provide a dielectric composition of the kind described in the opening paragraph which has a relative dielectric constant higher than 55, of which the TCC fulfills the NP0-standard, and which can be sintered with electrodes containing Cu.
It is a second object to provide a method of manufacturing a ceramic multilayer element of the kind described in the opening paragraph which has a high capacitance relative to its surface area, the temperature coefficient of said capacitance being low.
It is a third object of the invention to provide an electronic device as described in the opening paragraph which is suitable for high-frequency applications.